Ripple control has been used in the past to transmit information from a power source to a load and thus afford remote control of that load, for example, for control or water heaters, street lighting or the like.
Reference can be made to our Australian Patent No. 531592 (47714/79) (corresponding to UK No. 2022890, USA No. 4,359,644, Canada No. 1127710, Switzerland No. 633388 and other overseas patents) which described and claimed a system for the control of loads wherein sequential wave form distortion (SWD) signals were transmitted from a power sub-station to a consumer, a transmitter at the sub-station being coupled to the system to derive power from the system and generate a signal the frequency of which was synthesized from and locked to the power frequency, imposing the signal on the power frequency wave form as a sequence of wave form distortions, and the receiver was also coupled to the system and had a detector circuit comprising a pair of correlators (synchronous filters) driven 90.degree. out of phase with each other by a control frequency synthesized from and locked to the power frequency and having a frequency the same as the signal frequency, such that the signal input to the correlators appeared as a D.C. voltage output thereof, a squaring circuit was coupled to the signal output of each correlator and a summing circuit coupled to the squaring circuits, arranged so as to square and sum the D.C. voltage outputs of the correlators.
The differences between the sequential wave form distortion (SWD) control and conventional ripple control are that the signal was produced as a sequence of wave form distortions by applying a controlled oscillatory load to the power system, the frequency of the signal being locked to the supply frequency, and the synchronous correlation method of signal detection was used.
A series of tests have indicated that the main advantage of the SWD control over other (prior art) ripple control is its simplicity and lower cost, and also that most of the receiver circuit can be contained in a custom integrated circuit together with a microcomputer chip and no components are critical. The signal magnitude required for SWD control is about 10 to 20 percent of that required for ripple control and the reduced signal magnitude avoids many of the problems of interference with consumer apparatus sometimes experienced with ripple control.